This change adds the arenas package and a function to reflect for
allocating from an arena via reflection, but all the new API is placed
behind a GOEXPERIMENT.
For #51317.
Change-Id: I026d46294e26ab386d74625108c19a0024fbcedc
Reviewed-on: https://go-review.googlesource.com/c/go/+/423361 Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>